3 3 1 Introduction The quadratic assignment problem (QAP) was introduced by Koopmans and Beckmann in 1957 as a mathematical model for the location of a set of indivisible economical activities [113]. Consider the problem of allocating a set of facilities to a set of locations, with the cost being a function of the distance and flow between the facilities, plus costs associated with a facility being placed at a certain location. The objective is to assign each facility to a location such that the total cost is minimized. Specifically, we are given three n n input matrices with real elements F = (f ij ), D = (d kl ) and B = (b ik ), where f ij is the flow between the facility i and facility j, d kl is the distance between the location k and location l, and b ik is the cost of placing facility i at location k. The Koopmans-Beckmann version of the QAP can be formulated as follows: Let n be the number of facilities and locations and denote by N the set N = {1,2,...,n}. min φ S n i=1 j=1 f ij d φ(i)φ(j) + b iφ(i) (1) where S n is the set of all permutations φ : N N. Each individual product f ij d φ(i)φ(j) is the cost of assigning facility i to location φ(i) and facility j to location φ(j). In the context of facility location the matrices F and D are symmetric with zeros in the diagonal, and all the matrices are nonnegative. An instance of a QAP with input matrices F,D and B will be denoted by QAP(F,D,B), while we will denote an instance by QAP(F,D), if there is no linear term (i.e., B = 0). A more general version of the QAP was introduced by Lawler [118]. In this version we are given a four-dimensional array C = (c ijkl ) of coefficients instead of the two matrices F and D and the problem can be stated as min φ S n i=1 j=1 c ijφ(i)φ(j) + i=1 b iφ(i) (2) Clearly, a Koopmans-Beckmann problem QAP(F, D, B) can be formulated as a Lawler QAP by setting c ijkl := f ij d kl for all i,j,k,l with i j or k l and c iikk := f ii d kk + b ik, otherwise. Although extensive research has been done for more than three decades, the QAP, in contrast with its linear counterpart the linear assignment problem (LAP), remains one of the hardest optimization problems and no exact algorithm can solve problems of size n > 20 in reasonable computational time. In fact, Sahni and Gonzalez [164] have shown that the QAP is NP-hard and that even finding an approximate solution within some constant factor from the optimal solution cannot be done in polynomial time unless P=NP. These results hold even for the Koopmans-Beckmann QAP with coefficient matrices fulfilling the triangle inequality (see Queyranne [152]). So far only for a very special case of the Koopmans- Beckmann QAP, the dense linear arrangement problem a polynomial time approximation scheme has been found, due to Arora, Frieze, and Kaplan [7]. Complexity aspects of the QAP will be discussed in more detail in Section 3. Let us conclude this section with a brief review of some of the many applications of the QAP. In addition to facility layout problems, the QAP appears in applications such as backboard wiring, computer manufacturing, scheduling, process communications, turbine balancing, and many others. i=1

4 4 2 FORMULATIONS One of the earlier applications goes back to Steinberg [168] and concerns backboard wiring. Different devices such as controls and displays have to be placed on a panel, where they have to be connected to each other by wires. The problem is to find a positioning of the devices so as to minimize the total wire length. Let n be the number of devices to be placed and let d kl denote the wire length from position k to position l. The flow matrix F = (f ij ) is given by { 1 if device i is connected to device j, f ij = 0 otherwise. Then the solution to the corresponding QAP will minimize the total wire length. Another application in the context of location theory is a campus planning problem due to Dickey and Hopkins [58]. The problem consists of planning the sites of n buildings in a campus, where d kl is the distance from site k to site l, and f ij is the traffic intensity between building i and building j The objective is to minimize the total walking distance between the buildings. In the field of ergonomics Burkard and Offermann [36] showed that QAPs can be applied to typewriter keyboard design. The problem is to arrange the keys in a keyboard such as to minimize the time needed to write some text. Let the set of integers N = {1,2,...,n} denote the set of symbols to be arranged. Then f ij denotes the frequency of the appearance of the pair of symbols i and j. The entries of the distance matrix D = d kl are the times needed to press the key in position l after pressing the key in position k for all the keys to be assigned. Then a permutation φ S n describes an assignment of symbols to keys An optimal solution φ for the QAP minimizes the average time for writing a text. A similar application related to ergonomic design, is the development of control boards in order to minimize eye fatigue by McCormick [126]. There are also numerous other applications of the QAP in different fields e.g. hospital lay-out (Elshafei [63]), ranking of archeological data (Krarup and Pruzan [114]), ranking of a team in a relay race (Heffley [93]), scheduling parallel production lines (Geoffrion and Graves [76]), and analyzing chemical reactions for organic compounds (Ugi, Bauer, Friedrich, Gasteiger, Jochum, and Schubert [173]). 2 Formulations For many combinatorial optimization problems there exist different, but equivalent mathematical formulations, which stress different structural characteristics of the problem, which may lead to different solution approaches. Let us start with the observation that every permutation φ of the set N = {1,2,...,n} can be represented by an n n matrix X = (x ij ), such that { 1 if φ(i) = j, x ij = 0 otherwise. Matrix X is called a permutation matrix and is characterized by following assignment constraints x ij = 1, j = 1,2,...,n, i=1 x ij = 1, j=1 x ij {0,1}, i = 1,2,...,n, i,j = 1,2,...,n.

5 2.1 Quadratic Integer Program Formulation 5 We denote the set of all permutation matrices by X n. Due to a famous theorem of Birkhoff the permutation matrices correspond in a unique way to the vertices of the assignment polytope ( the Birkhoff polytope, the perfect matching polytope of K n,n etc.). This leads to the following description of a QAP as quadratic integer program. 2.1 Quadratic Integer Program Formulation Using permutation matrices instead of permutations, the QAP ((2) can be formulated as the following integer program with quadratic objective function (hence the name Quadratic Assignment Problem by Koopmans and Beckmann [113]). min s.t. c ijkl x ik x jl + b ij x ij (3) i=1 j=1 k=1 l=1 i,j=1 x ij = 1, j = 1,2,...,n, (4) i=1 x ij = 1, i = 1,2,...,n, (5) j=1 x ij {0,1}, i,j = 1,2,...,n. (6) From now on, whenever we write (x ij ) X n, it will be implied that the x ij satisfy the assignment constraints (4), (5) and (6). Many authors have proposed methods for linearizing the quadratic form of the objective function (3) by introducing additional variables; some of these of linearizations will be discussed in Section 4. A QAP in Koopmans-Beckmann form can be formulated in a more compact way if we define an inner product between matrices. Let the inner product of two real n n matrices A,B be defined by A,B := a ij b ij. i=1 j=1 Given some n n matrix A, a permutation φ S n and the associated permutation matrix X X n, then AX T and XA permute the columns and rows of A, respectively, according to the permutation φ and therefore XAX T = (a φ(i)φ(j) ). Thus we can formulate a Koopmans-Beckmann QAP alternatively as min F,XDX T + B,X (7) s.t. X X n. 2.2 Concave Quadratic Formulation In the objective function of (3), let the coefficients c ijkl be the entries of an n 2 n 2 matrix S, such that c ijkl is on row (i 1)n + k and column (j 1)n + l. Now let Q := S αi, where I is the (n 2 n 2 ) unit matrix and α is greater than the row norm S of matrix S. The subtraction of a constant from the entries on the main diagonal of S does not change the optimal solutions of the corresponding QAP, it simply adds a constant to the

7 2.4 Kronecker Product 7 since d ji = d φ(i)φ(j), i,j = 1,...,n, where φ S n is the permutation associated with X (see 2.1). Since tr(bx T ) = n i=1 b iφ(i), the QAP in (7) can be formulated as min tr(fxd T + B)X T (9) s.t. X X n. The trace formulation of the QAP first appeared in Edwards [61, 62], and was used by Finke, Burkard, and Rendl [67] to introduce the eigenvalue lower bounding techniques for symmetric QAPs (see Section 7.1). Given any two real n n matrices A, B, recall the well known properties tr(ab) = tr(ba), (AB) T = B T A T and tra = tra T. For F = F T we can then write the quadratic term in (9) as trfxd T X T = trfxdx T, where D is not necessarily symmetric. Therefore, given a QAP instance where only one of the matrices is symmetric (say F), we can transform it into a QAP instance where both matrices are symmetric. This is done by introducing a new symmetric matrix E = 1 2 (D + DT ): trfxe T X T = 1 2 tr(fxdt X T + FXDX T ) = trfxd T X T. 2.4 Kronecker Product Let A be a real m n matrix and let B be a real p q matrix. Then the Kronecker product of matrices A and B is defined as a 11 B a 12 B a 1n B A B := a m1 B a m2 B a mn B That is, A B is the mp nq matrix formed from all possible pairwise element products of A and B. If we let vec(x) IR n2 be the vector formed by the columns of a permutation matrix X, the QAP can be formulated as min vec(x) T (F D)vec(X) + vec(b) T vec(x), (10) s.t. X X n. Operations using the Kronecker product and its properties have been studied in detail by Graham [84]. However, the above formulation is rarely used in investigations of the QAP. Based on that formulation Lawler [118] gave an alternative formulation of the QAP as a linear assignment problem (LAP) of size n with the additional constraint that only (n 2 n 2 ) permutation matrices which are Kronecker products of n n permutation matrices are feasible. If as before the (n 2 n 2 ) cost matrix C contains the n 4 costs c ijkl, such that the (ijkl)-th element corresponds to the element in the ((i 1)n+k)-th row and ((j 1)n+l)-th column of C, the QAP can be written as min C,Y s.t. Y = X X, (11) X X n. Because of the additional constraint to be fulfilled by the feasible solutions the resulting LAP cannot be solved efficiently.

8 8 3 COMPUTATIONAL COMPLEXITY 3 Computational complexity The results described in this section bring evidence to the fact that the QAP is a very hard problem from the theoretical point of view. Not only that the QAP cannot be solved efficiently but it even cannot be approximated efficiently within some constant approximation ratio. Furthermore, finding local optima is not a trivial task even for simply structured neighborhoods like the 2-opt neighborhood. Two early results obtained by Sahni and Gonzalez [164] in 1976 settled the complexity of solving and approximating the QAP. It was shown that the QAP is NP-hard and that even finding an ǫ-approximate solution for the QAP is a hard problem, in the sense that the existence of a polynomial ǫ-approximation algorithm implies P = N P. In the following, let Z(F,D,φ) denote the objective function value of a solution φ for a QAP with flow matrix F and distance matrix D. Definition 3.1 Given a real number ǫ > 0, an algorithm Υ for the QAP is said to be an ǫ-approximation algorithm if Z(F,D,π Υ ) Z(F,D,π opt ) Z(F,D,π opt ) ǫ, (12) holds for every instance QAP(F,D), where π Υ is the solution of QAP(F,D) computed by algorithm Υ and π opt is an optimal solution of QAP(F,D). The solution of QAP(F,D) produced by an ǫ-approximation algorithm is called an ǫ-approximate solution. Theorem 3.2 (Sahni and Gonzalez [164], 1976) The quadratic assignment problem is strongly NP-hard. For an arbitrary ǫ > 0, the existence of a polynomial time ǫ-approximation algorithm for the QAP implies P = N P. The proof is done by a reduction from the Hamiltonian cycle problem: Given a graph G, does G contain a cycle which visits each vertex exactly once (see [73])? Queyranne [152] derives an even stronger result which further confirms the widely spread belief on the inherent difficulty of the QAP in comparison with other difficult combinatorial optimization problems. It it well known and very easy to see that the traveling salesman problem (TSP) is a special case of the QAP. The TSP on n cities can be formulated as a QAP(F,D) where F is the distance matrix of the TSP instance and D is the adjacence matrix of a Hamiltonian cycle on n vertices. In the case that the distance matrix is symmetric and satisfies the triangle inequality, the TSP is approximable in polynomial time within 3/2 as shown by Christofides [46]. Queyranne [152] showed that, unless P = N P, QAP(A, B) is not approximable in polynomial time within some finite approximation ratio, even if A is the distance matrix of some set of points on a line and B is a symmetric block diagonal matrix. A more recent result of Arora, Frieze and Kaplan [7] answers partially one of the open questions stated by Queyranne in [152]. What happens if matrix A is the distance matrix of n points which are regularly spaced on a line, i.e., points with abscissae given by x p = p, p = 1,...,n? This special case of the QAP is termed linear arrangement problem and is a well studied NP-hard problem. In the linear arrangement problem the matrix B is not restricted to have the block diagonal structure mentioned above, but is simply a symmetric 0-1 matrix. Arora et al. give a polynomial time approximation scheme (PTAS) for the

9 9 linear arrangement problem in the case that the 0-1 matrix B is dense, i.e., the number of 1 entries in B is in Ω(n 2 ), where n is the size of the problem. They show that for each ǫ > 0 there exists an ǫ-approximation algorithm for the dense linear arrangement problem with time complexity depending polynomially on n and exponentially on 1/ǫ, hence polynomial for each fixed ǫ > 0. Recently it has been shown that even finding a locally optimal solution of the QAP can be prohibitively hard, i.e., even local search is hard in the case of the QAP. Below we formalize this idea to some extent. Assume that an optimization problem P is given by specifying a ground set E, a set F 2 E of feasible solutions and a cost function c: E IR. This cost function c implies an objective function f : F IR defined by f(s) = x S c(x), for all S F. The goal is to find a feasible solution which minimizes the objective function. For every feasible solution S F let a neighborhood N(S) F of S be given. This neighborhood consists of feasible solutions which are somehow close to S. Now, instead of looking for a globally optimal solution S F of the problem P, that is f(s ) = min S F f(s), we look for a locally optimal solution or a local minimum of P, that is an S F such that f( S) = min f(s). S N( S) An algorithm which produces a locally optimal solution, is frequently called a local search algorithm. Some local search algorithms for the QAP are described in Section 8. Let us consider the intriguing question Is it easy to find a locally optimal solution for the QAP?. Clearly the answer depends on the involved neighborhood structure. If the neighborhoods N(S) are replaced by new neighborhoods N (S), one would generally expect changes in the local optimality status of a solution. The theoretical basis for facing this kind of problems was introduced by Johnson, Papadimitriou and Yannakakis in [97]. They define the so-called polynomial-time local search problems, shortly PLS problems. A pair (P, N), where P is a (combinatorial) optimization problem P and N is an associated neighborhood structure, defines a local search problem which consists of finding a locally optimal solution of P with respect to the neighborhood structure N. Without going into technical details a PLS problem is a local search problem for which local optimality can be checked in polynomial time. In analogy with decision problems, there exist complete problems in the class of PLS problems. The PLS-complete problems, are in the usual complexity sense the most difficult among the PLS problems. Murthy, Pardalos and Li [138] introduce a neighborhood structure for the QAP which is similar to the neighborhood structure proposed by Kernighan and Lin [109] for the graph partitioning problem. For this reason we will call it a K-L type neighborhood structure for the QAP. Murthy et al. show that the corresponding local search problem is PLS-complete. A K-L type neighborhood structure for the QAP. Consider a permutation φ 0 S n. A swap of φ 0 is a permutation φ S n obtained from φ 0 by applying a transposition (i,j) to it, φ = φ 0 (i,j). A transposition (i,j) is defined as a permutation which maps i to j, j to i, and k to k for all k {i,j}. In the facility location context a swap is obtained by interchanging the facilities assigned to two locations i and j. A greedy swap of permutation

10 10 4 LINEARIZATIONS φ 0 is a swap φ 1 which minimizes the difference Z(F,D,φ) Z(F,D,φ 0 ) over all swaps φ of φ 0. Let φ 0,φ 1,...,φ l be a set of permutations in S n, each of them being a greedy swap of the preceding one. Such a sequence is called monotone if for each pair of permutations φ k, φ t in the sequence, {i k,j k } {i t,j t } =, where φ k (π t ) is obtained by applying transposition (i k,j k ) ((i t,j t )) to the preceding permutation in the sequence. The neighborhood of φ 0 consists of all permutations which occur in the (unique) maximal monotone sequence of greedy swaps starting with permutation φ 0. Let us denote this neighborhood structure for the QAP by N K-L. It is not difficult to see that, given a QAP(F,D) of size n and a permutation φ S n, the cardinality of N K-L (π) does not exceed n/ It is easily seen that the local search problem (QAP, N K-L ) is a PLS problem. Pardalos, Rendl, and Wolkowicz [147] have shown that a PLS-complete problem, namely the graph partitioning problem with the neighborhood structure defined by Kernighan and Lin [109] is PLS-reducible to (QAP, N K L ). This implies the following result. Theorem 3.3 (Pardalos, Rendl and Wolkowicz [147], 1994) The local search problem (QAP, N K-L ), where N K-L is the Kernighan-Lin type neighborhood structure for the QAP, is PLS-complete. The PLS-completeness of (QAP, N K-L ) implies that, in the worst case, a general local search algorithm as described above involving the Kernighan-Lin type neighborhood finds a local minimum only after a time which is exponential on the problem size. Numerical results, however, show that such local search algorithms perform quite well when applied to QAP test instances, as reported in [138]. Another simple and frequently used neighborhood structure in S n is the so-called pairexchange (or 2-opt) neighborhood N 2. The pair-exchange neighborhood of a permutation φ 0 S n consists of all permutations φ S n obtained from φ 0 by applying some transposition (i,j) to it. Thus, N 2 (φ) = {φ (i,j): 1 i,j n, i j, }. It can also be shown that (QAP, N 2 ) is PLS-complete. Schäffer and Yannakakis [165] have proven that the graph partitioning problem with a neighborhood structure analogous to N 2 is PLS-complete. A similar PLS-reduction as in [147] implies that the local search problem (QAP, N 2 ), where N 2 is the pair-exchange neighborhood, is PLS-complete. This implies that the time complexity of a general local search algorithm for the QAP involving the pair-exchange neighborhood is also exponential in the worst case. Finally, let us mention that no local criteria are known for deciding how good a locally optimal solution is as compared to a global one. From the complexity point of view, deciding whether a given local optimum is a globally optimal solution to a given instance of the QAP, is a hard problem, see Papadimitriou and Wolfe [145]. 4 Linearizations The first attempts to solve the QAP eliminated the quadratic term in the objective function of (2), in order to transform the problem into a (mixed) 0-1 linear program. The linearization of the objective function is usually achieved by introducing new variables and new linear (and binary) constraints. Then existing methods for (mixed) linear integer programming (MILP) can be applied. The very large number of new variables and constraints, however, usually poses an obstacle for efficiently solving the resulting linear integer programs. MILP formulations provide moreover LP relaxations of the problem which can be used to compute lower bounds. In this context the tightness of the continuous relaxation of the resulting linear integer program is a desirable property.

11 4.1 Lawler s Linearization 11 In this section we present four linearizations of the QAP: Lawler s linearization [118], which was the first, Kaufmann and Broeckx s linearization [108], which has the smallest number of variables and constraints, Frieze and Yadegar s linearization [70] and the linearization of Adams and Johnson [3]. The last linearization which is a slight but relevant modification of the linearization proposed by Frieze and Yadegar [70], unifies most of the previous linearizations and is important for getting lower bounds. 4.1 Lawler s Linearization Lawler [118] replaces the quadratic terms x ij x kl in the objective function of (2) by n 4 variables y ijkl := x ij x kl, i,j,k,l = 1,2,...,n, and obtains in this way a 0-1 linear program with n 4 +n 2 binary variables and n 4 +2n 2 +1 constraints. Thus the QAP can be written as the following 0-1 linear program (see [118, 23]) min i,j=1 k,l=1 c ijkl y ijkl s.t. (x ij ) X n, y ijkl = n 2, (13) i,j=1 k,l=1 x ij + x kl 2y ijkl 0, y ijkl {0,1}, i,j,k,l = 1,2,...,n, i,j,k,l = 1,2,...,n. 4.2 Kaufmann and Broeckx Linearization By adding a large enough constant to the cost coefficients, which does not change the optimal solution, we may assume that all cost coefficients c ijkl are nonnegative. By rearranging terms in the objective function (2) we obtain x ij i,j=1 k,l=1 Kaufmann and Broeckx [108] define n 2 new real variables w ij := x ij c ijkl x kl. (14) c ijkl x kl, i,j = 1,...,n, (15) k,l=1 and plug them in the objective function of (14) to obtain a linear objective function of the form w ij. i,j=1

13 4.4 Adams and Johnson Linearization 13 It is interesting to notice here that the gap between the optimal value of this continuous relaxation and the optimal value of the QAP can be enormous. Dyer, Frieze, and Mc- Diarmid [60] showed for QAPs whose coefficients c ijkl are independent random variables uniformly distributed on [0, 1] that the expected optimal value of the above mentioned linearization has a size of O(n). On the other hand the expected optimal value of such QAPs increases with high probability as Ω(n 2 ), as shown by Burkard and Fincke [32]. Consequences of this asymptotic behavior will be discussed in some detail in Section 12. No similar asymptotic result is known for the continuous relaxation of the linearization due to Adams and Johnson [3] which is presented in the following section. 4.4 Adams and Johnson Linearization Adams and Johnson presented in [3] a new 0-1 linear integer programming formulation for the QAP, which resembles to a certain extent the linearization of Frieze and Yadegar. It is based on the linearization technique for general 0-1 polynomial programs introduced by Adams and Sherali in [4, 5]. The QAP with array of coefficients C = (c ijkl ) is proved to be equivalent to the following mixed 0-1 linear program min c ijkl y ijkl (25) i,j=1 k,l=1 s.t. (x ij ) X n, y ijkl = x kl, j,k,l = 1,...,n, i=1 y ijkl = x kl, i,k,l = 1,2,...,n, j=1 y ijkl = y klij, i,j,k,l = 1,...,n, (26) y ijkl 0, i,j,k,l = 1,2,...,n, where each y ijkl represents the product x ij x kl. The above formulation contains n 2 binary variables x ij, n 4 continuous variables y ijkl, and n 4 + 2n 3 + 2n constraints excluding the nonnegativity constraints on the continuous variables. Although as noted by Adams and Johnson [3] a significant smaller formulation in terms of both the variables and constraints could be obtained, the structure of the continuous relaxation of the above formulation is favorable for solving it approximately by means of the Lagrangean dual. (See Section 6.2 for more information.) The theoretical strength of the linearization (25) lies in the fact that the constraints of the continuous relaxations of previous linearizations can be expressed as linear combinations of the constraints of the continuous relaxation of (25), see [3, 98]. Moreover, many of the previously published lower-bounding techniques can be explained based on the Lagrangean dual of this relaxation. For more details on this topic we refer to Section 6.2. As noted by the Adams et al. [3], the constraint set of (25) describes a solution matrix Y which is the Kronecker product of two permutation matrices (i.e., Y = X X where X S n ), and hence this formulation of the QAP is equivalent to (11).

14 14 5 QAP POLYTOPES 5 QAP Polytopes A polyhedral description of the QAP and of some of his relatives have been recently investigated by Barvinok [12], Jünger and Kaibel [100, 101], Kaibel [102], and Padberg and Rijal [142, 161]. Although in an early stage yet, the existing polyhedral theory around the QAP counts already a number of results concerning basic features like dimensions, affine hulls, and valid and facet defining inequalities for the general QAP polytope and the symmetric QAP polytope. The linearization of Frieze and Yadegar introduced in the previous section can be used as a starting point for the definition of the QAP polytope. The QAP polytope is defined as a convex hull of all 0-1 vectors (x ij,y ijkl ), 1 i,j,k,l n, which are feasible solutions of the MILP formulation of Frieze and Yadegar [70]. Another possibility to introduce the QAP polytope is the formulation of the QAP as a graph problem as proposed by Jünger and Kaibel [100]. This formulation provides some additional insight in the problem and allows an easier use of some technical tools e.g. projections and affine transformations. The latter lead to a better understanding of the relationship between the general QAP polytope and related polytopes, e.g. the symmetric QAP polytope, or well studied polytopes of other combinatorial optimization problems like the traveling salesman polytope or the cut polytope (see [102]). For each n IN consider a graph G n = (V n,e n ) with vertex set V n = {(i,j): 1 i,j n} and edge set E n = {((i,j),(k,l)): i k,j l}. Clearly, the maximal cliques in G n have cardinality n and correspond to the permutation matrices. Given an instance of the Lawler QAP with coefficients c ijkl and linear term coefficients b ij, we introduce b ij as vertex weights and c ijkl as weight of the edge ((i,j),(k,l)). Solving the above QAP instance is equivalent to finding a maximal clique with minimum total vertex- and edge-weight. For each clique C in G n with n vertices we denote its incidence vector by (x C,y C ), where x C IR n2, y C IR n2 (n 1) 2 2 x ij = { 1 if (i,j) C, 0 otherwise y ijkl = { 1 if (i,j),(k,l) C, 0 otherwise The QAP polytope denoted by QAP n is then given by QAP n := conv{(x C,y C ): C is a clique with n vertices in G n }. It turns out that the traveling salesman polytope and the linear ordering polytope are projections of QAP n, and that QAP n is a face of the Boolean quadric polytope, see [102]. Barvinok [12], Padberg and Rijal [142], and Jünger and Kaibel [100] have independently computed the dimension of QAP n, and have shown that the inequalities y ijkl 0, i k, j l, are facet defining. (These are usually called trivial facets of QAP n.) Moreover, Padberg and Rijal [142], and Jünger and Kaibel [100] have independently shown that the

15 15 affine hull of QAP n is described by the following equations which are linearly independent: k 1 x kl + y ijkl + i=1 j 1 x ij + y ijkl + l=1 x ij = 1, 1 j n 1 (27) i=1 x ij = 1, 1 i n, (28) j=1 i=k+1 l=j+1 y klij = 0 y ijkl = 0 1 j l n,1 k n 1, or 1 l < j n,k = n 1 j n,1 i n 3, i < k n 1 or 1 j n 1,i = n 2, k = n 1 (29) (30) x kj + j1 y ilkj + l1 l=j+1 y ilkj = 0 1 j n 1,1 i n 3, i < k n 1 (31) Summarizing we get the following theorem: Theorem 5.1 (Barvinok [12], 1992, Jünger and Kaibel [100], 1996, Padberg and Rijal [142], 1996) (i) The affine hull of the QAP polytope QAP n is given by the linear equations (27)-(31). These equations are linearly independent and the rank of the system is 2n(n 1) 2 (n 1)(n 2), for n 3. (ii) The dimension of QAP n is equal to 1+(n 1) 2 +n(n 1)(n 2)(n 3)/2, for n 3. (iii) The inequalities y ijkl 0, i < k, j l, define facets of QAP n. Padberg and Rijal [142] identified additionally two classes of valid inequalities for QAP n, the clique inequalities and the cut inequalities, where the terminology is related to the graph G n. The authors identify some conditions under which the cut inequalities are not facet defining. It is an open problem, however, to identify facet defining inequalities within these classes. A larger class of valid inequalities, the so-called box inequalities have been described by Kaibel [102]. Those inequalities are obtained by exploiting the relationship between the Boolean quadric polytope and the QAP polytope. A nice feature of the box inequalities is that it can be decided efficiently whether they are facet defining or not, and in the latter case some facet defining inequality which dominates the corresponding box inequality can be derived. Similar results have been obtained for the symmetric QAP polytope, SQAP n, arising in the case that at least one of the coefficient matrices of the given QAP (matrices F, D in (1)) is symmetric. The definition of SQAP n is given by means of a hypergraph H n = (V n,f n ), where V n is the same set of vertices as in graph G n and F n is the set of hyperedges {(i,j),(k,l),(i,l),(k,j)} for all i k, j l. A set C V n is called a clique in

17 17 are valid for SQAP n. All curtain inequalities with 3 I, J n 3 define facets of SQAP n. The other curtain inequalities define faces which are contained in trivial facets of SQAP n. Finally, there are some additional results concerning the affine description and the facial structure of polytopes of special versions of sparse QAPs, e.g. sparse Koopmans-Beckmann QAPs, see Kaibel [102]. The idea is to take advantage of the sparsity for a better analysis and description of the related polytopes. These investigations, however, are still in their infancy. 6 Lower Bounds Lower bounding techniques are used within implicit enumeration algorithms, such as branch and bound, to perform a limited search of the feasible region of a minimization problem, until an optimal solution is found. A more limited use of lower bounding techniques concerns the evaluation of the performance of heuristic algorithms by providing a relative measure of proximity of the suboptimal solution to the optimum. In comparing lower bounding techniques, the following criteria should be taken into consideration: Complexity of computing the lower bound. Tightness of the lower bound (i.e., small gap between the bound and the optimum solution). Efficiency in computing lower bounds for subsets of the original feasible set. Since there is no clear ranking of the performance of the lower bounds that will be discussed below, all of the above criteria should be kept in mind while reading the following paragraphs. Considering the asymptotic behavior of the QAP (see Section 12) it should be fair to assume that the tightness of the lower bound probably dominates all of the above criteria. In other words, if there is a large number of feasible solutions close to the optimum, then a lower bound which is not tight enough, will fail to eliminate a large number of subproblems in the branching process. 6.1 Gilmore-Lawler Type Lower Bounds Based on the formulation of the general QAP as an LAP of dimension n 2 stated in formulation (11), Gilmore [77] and Lawler [118] derived lower bounds for the QAP, by constructing a solution matrix Y in the process of solving a series of LAPs. If the resulting matrix Y is a permutation matrix, then the objective function value yielded by Y is optimal, otherwise it is bounded from below by C,Y. In this section we briefly describe a number of bounding procedures which exploit this basic idea. The Gilmore-Lawler bound Consider an instance of the Lawler QAP (2) with coefficients C = (c ijkl ), and partition the array C into n 2 matrices of dimension n n, C (i,j) = (c ijkl ), for each fixed pair (i,j), i,j = 1,2,...,n. Each matrix C (i,j) essentially contains the costs associated with the assignment x ij = 1. Partition the solution array Y = (y ijkl ) also into n 2 matrices, Y (i,j) = (y ijkl ), for fixed i,j = 1,2,...,n.

18 18 6 LOWER BOUNDS For each pair (i,j), 1 i,j n, solve the LAP with cost matrix C (i,j) and denote its optimal value by l ij : l ij = min s.t. c ijkl y ijkl (36) k=1 l=1 y ijkl = 1, k=1 y ijkl = 1, l=1 l = 1,2,...,n, k = 1,2,...,n, y ijij = 1 (37) y ijkl {0,1}, i,j = 1,2,...,n. (38) Observe that constraint (37) essentially reduces the problem into an LAP of dimension (n 1) with cost matrix obtained from C (i,j) by deleting its i-th row and j-th column. For each i,j, denote by Y (i,j) the optimal solution matrix of the above LAP. The Gilmore-Lawler lower bound GLB(C) for the Lawler QAP with coefficient array C is given by the optimal value of the LAP of size n with cost matrix (l ij ) GLB(C) = min i=1 j=1 s.t. (x ij ) X n. l ij x ij (39) Denote by X = (x ij ) the optimal solution matrix of this last LAP. If 1 n ij x ij Y (ij) X n, then the array Y = (yijkl (i,j) ) with matrices Y = x ij Y (ij) for all i,j, 1 i,j n, is a Kronecker product of two permutation matrices of dimension n, and hence an optimal solution of the considered QAP. Since each LAP can be solved in O(n 3 ) time, the above lower bound for the Lawler QAP (2) of dimension n can be computed in O(n 5 ) time. For the more special Koopmans-Beckmann QAP (1), where the quadratic costs c ijkl are given as entry-wise products of two matrices F = (f ij ) and D = (d ij ), c ijkl = f ij d kl for all i,j,k,l, the computational effort can be reduced to O(n 3 ). This is due to the following well known result of Hardy, Littlewood, and Pólya [92]: Proposition 6.1 (Hardy, Littlewood and Pólya [92], 1952) Given two n-dimensional real vectors a = (a i ), b = (b i ) such that 0 a 1 a 2... a n and b 1 b 2... b n 0, the following inequalities hold for any permutation φ of 1,2,...,n: a i b i a i b φ(i) a i b n i+1 i=1 i=1 Given two arbitrary nonnegative vectors a,b IR n, let φ be a permutation which sorts a non-decreasingly and ψ a permutation which sorts a non-increasingly. Moreover, let π be a permutation which sorts b non-increasingly. We denote i a,b := a φ(i) b π(i) a,b + := i=1 a ψ(i) b π(i) (40) i=1

19 6.1 Gilmore-Lawler Type Lower Bounds 19 Consider now an instance (1) of the Koopmans-Beckmann QAP. This can be written as a Lawler QAP of the form (2) by setting { fik d jl, for i k,j l c ijkl := f ii d jj + b ij, for i = k,j = l. Each matrix C (i,j) of the array C is then given by C (i,j) = (f ik d jl ). Therefore, instead of solving n 2 LAPs we can easily compute the values l ij by applying Proposition 6.1, as l ij = f ii d jj + b ij + ˆf (i,.), ˆd (j,.), (41) where ˆf (i,.), ˆd (j,.) IR n 1 are (n 1)-dimensional vectors obtained from the i-th and the j-th row of F and D by deleting the i-th and the j-th element, respectively. Finally, by solving the LAP with cost matrix (l ij ) as in (39), we obtain the Gilmore-Lawler lower bound for the Koopmans-Beckman QAP. The appropriate sorting of the rows and columns of F and D can be done in O(n 2 log n) time. Then the computation of all l ij takes O(n 3 ) time and the same amount of time is needed to solve the last LAP. Similar bounds have been proposed by Christofides and Gerrard [48]. The basic idea relies again on decomposing the given QAP into a number of subproblems which can be solved efficiently. First solve each subproblem, then build a matrix with the optimal values of the subproblems, and solve an LAP with that matrix as cost matrix to obtain a lower bound for the given QAP. Christofides et al. decompose the Koopmans-Beckmann QAP(F, D) based on isomorphic-subgraphs of graphs whose weighted adjacency matrices are F and D. The GLB is obtained as a special case, if these subgraphs are stars, and it generally outperforms the bounds obtained by employing other subgraphs, like single edges, or double stars (see also [74]). The Gilmore-Lawler bound is simple to compute, but it deteriorates fast as n increases. The quality of this lower bound can be improved if the given problem is transformed such that the contribution of the quadratic term in the objective function is decreased by moving costs to the linear term. This is the aim of the so-called reduction methods. Reduction methods Consider a Lawler QAP as in (2), and assume that b ij = 0 for all i,j. By the above discussion the GLB will be given as solution of the following LAP min (l ij + c ijij )x ij i=1 j=1 s.t. (x ij ) X n. (42) We want to decompose the cost coefficients in the quadratic term of (2) and transfer some of their value into the linear term such that c ijij l ij. This would yield a tighter lower bound because the LAP can be solved exactly. This procedure is known as reduction and was introduced by Conrad [54]. Reductions have been investigated by many researchers (see [21, 162, 62, 70]). The general idea is to decompose each quadratic cost coefficient into several terms so as to guarantee that some of them end up in being linear cost coefficients and can be moved in the linear term of the objective function. Consider the following general decomposition scheme: D-1: c ijkl = c ijkl + e ijk + g ijl + h ikl + t jkl, i k, j l,

20 20 6 LOWER BOUNDS where e,g,h,t IR n3. Substituting the above in the objective function of (2) we obtain a new QAP which is equivalent with the given one and whose objective function has a quadratic and a linear part. (Formulas for the coefficients of this new QAP can be found in the literature, e.g. [70].) For the quadratic term we can compute the Gilmore-Lawler bound. Then we add it to the optimal value of the linear part in order to obtain a lower bound for the QAP. In the case of the Koopmans-Beckman QAP the general decomposition scheme is where λ,µ,ν,φ IR n. D-2: f ij = f ij + λ i + µ j, i j, d kl = d kl + ν k + φ l, k l, Frieze and Yadegar [70] have shown that the inclusion of vectors h and t in D-1, or similarly the inclusion of vectors µ and φ in D-2, does not affect the value of the lower bound. Therefore these vectors are redundant. As mentioned also in Section 4.3, Frieze and Yadegar derived lower bounds for the QAP based on a Lagrangean relaxation of the mixed integer linear programming formulation (17)-(24). By including the constraints (19) and (20) in the objective function (17) and using vectors e and g as Lagrangean multipliers, we get the following Lagrangean problem L(e, g) = min { ijkl c ijkly ijkl + jkl e jkl (x kl i y ijkl) + ikl g ikl ijkl (c ijkl e jkl g ikl )y ijkl + ij ( k e kij + l g lij) x ij s.t. constraints (18), (21),...,(24). ( x kl )} j y ijkl = As proved in [70], for any choice of e and g, the solution to the above Lagrangean problem equals the value of the GLB obtained after the decomposition of the coefficient c ijkl by using only vectors e and g in D-1. Therefore, max e,g L(e,g) constitutes a lower bound for the QAP which is larger (i.e., better) than all GLBs obtained after applying reduction methods according to D-1 (D-2). Frieze and Yadegar propose two subgradient algorithms to approximately solve max e,g L(e,g), and obtain two lower bounds, denoted by FY 1 and F Y 2. These bounds seem to be sharper than the previously reported Gilmore-Lawler bounds obtained after applying reductions. Bounding techniques based on reformulations Consider the Lawler QAP with a linear term in the objective function: min s.t. c ijkl x ik x jl + n b ik x ik i,k=1 j,l=1 i,k=1 x ik = 1, 1 k n, x ik = 1, 1 i n, i=1 k=1 x ik {0,1}, 1 i,k n.

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